In oil producing wells, a production tubing string extends centrally through the well casing to the lower depths of the well where oil is present. Sucker rods pass through the pipe string to an appropriate down hole pump so that oil is then brought up through the production tubing string to the surface of the well.
The temperature in the lower portions of the well is sufficiently high that wax, such as parafin, will not solidify out of the petroleum products being pumped to the surface. However, such solidification can and often does occur when cooler regions of the well are reached. The solidification of the wax results in a build up of wax on the interior of the production string at these higher cooler levels and impedes the proper flow of oil.
In view of the foregoing, it is common practice to circulate down the production tubing a hot fluid such as very hot oil which serves to melt any built up wax or parafin. In this respect, there is provided a special valve sub in the lower portion of the production tubing string to pass the hot dewaxing oil from the interior of the tubing to the well annulus; that is, to the exterior of the remaining portion of the tubing through which it is not necessary to pass the hot dewaxing fluid.
The particular valves presently used are hydraulically operated; that is, they usually comprise a valve seat and spring biased ball wherein the ball is located on the downstream side of the valve seat; that is, the side of the valve seat communicating with the well annulus. When a sufficient hydraulic pressure of the hot dewaxing fluid is built up, it will simply force the ball off of the valve seat against the bias of the spring so that the dewaxing fluid can escape to the annulus.
Typical prior art dewaxing valves of the foregoing type are shown, by way of example, in U.S. Pat. Nos. 2,300,348; 3,014,531; and 4,011,906.
Basically, two problems arise with valves of the above type. First, should the biasing spring for the ball fail, the ball will drop away from the valve seat and all of the oil within the production tubing will be lost into the annulus. Second, since the ball is unseated from the valve seat by hydraulic pressure of the dewaxing fluid against the bias of the springs, a relatively high pressure is required to initially unseat the ball. Thereafter, when flow occurs around the ball past the seat, there is a drop in pressure on the ball and the spring can reseat the ball, setting up a "chattering" situation. Of course, the very high pressure can be maintained even under flowing conditions but the result is fairly unsatisfactory, erratic flow still occurring and the flow rate being so fast that proper dewaxing cannot always be realized.